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1.
One of the largest earthquakes ever recorded in Africa (surface wave magnitude M(s) = 7.2) occurred about 50 kilometers east of the Upper River Nile on 20 May 1990. Four days later, two more large earthquakes (M(s) = 6.4 and 7.0) occurred about 50 kilometers to the northwest in the Nile Valley. In the following months, a further 60 events were recorded by seismic stations worldwide. The earthquakes are associated with two fault systems: one east of the Nile with azimuth southeast and one along the Nile Valley with azimuth north-northeast. The activity alternated between the two fault systems and indicates that the northern extremity of the western branch of the East African Rift System extends at least 350 kilometers north of Lake Albert. 相似文献
2.
Hooper PR 《Science (New York, N.Y.)》1982,215(4539):1463-1468
Between 17 million and 6 million years ago, 200,000 square kilometers of the American Northwest were flooded by basaltic lava that erupted through fissures in the crust up to 150 kilometers long. Larger individual eruptions covered over a third of the Columbia Plateau in a few days. The lavas represent partial melts of the earth's mantle that were only slightly modified by near-surface, upper crustal processes. The abundant chemical and mineralogical data now available offer an opportunity to study mantle composition and the processes involved in the evolution of the earth's crust. 相似文献
3.
Hynds RJ Cowley SW Sanderson TR Wenzel KP VAN Rooijen JJ 《Science (New York, N.Y.)》1986,232(4748):361-365
During the encounter with comet Giacobini-Zinner, the energetic particle anisotropy spectrometer on the International Cometary Explorer spacecraft observed large fluxes of energetic ions, believed to result principally from ionization of the cometary atmosphere followed by pickup and acceleration by the ambient flow of the solar wind. These heavy cometary ions were observed from approximately 1 day before closest approach to about 2(1/2) days afterward. Three regimes of differing ion characteristics have been identified. An outer region with a scale of approximately 10(6) kilometers contains variable fluxes of antisolar-streaming pick-up ions in the undisturbed solar wind. In the middle region, of approximately 10(5) kilometers, fluxes have less large-scale variability and broader angular and energy distributions. This region is separated from the outer zone by a sharp transition. The inner region has a scale of approximately 10(4) kilometers and is characterized by reduced fluxes and complex angular distributions. 相似文献
4.
Measurements of satellite drag obtained from the orbital decay of the Pioneer Venus orbiter on the nightside of Venus indicate an atomic oxygen atmosphere near 155 kilometers (an order of magnitude less dense than expected) with nighttime inferred exospheric temperatures averaging as low as 110 K. Densities at these altitudes decrease sharply from day to night, contrary to the predicted nighttime oxygen bulge. This decrease may be indicative of an unexpectedly weak transport across the evening terminator or a very strong heat sink at night that is possibly related to vertical eddy heat transport. Large periodic oscillations in density and inferred exospheric temperature are detected with a period of 5 to 6 days. We have subsequently discovered temperature variations of the same period in the stratosphere, which are tentatively interpreted as planetary-scale waves that may propagate upward producing the periodic variations in the thermosphere and exosphere. The peak-to-peak amplitude of the temperature oscillations associated with these waves apparently increases with altitude approximately as follows: 1 K (70 kilometers), 3 K (90 kilometers), 40 K (155 kilometers). Inferred nighttime exospheric temperatures are found to be asymmetric relative to midnight, minimizing on the morning side. The possibility of superrotation of the thermosphere, and exosphere is discussed. 相似文献
5.
Small-magnitude earthquakes began beneath Mount St. Helens 40 days before the eruption of 20 March 1982. Unlike earlier preeruption seismicity for this volcano, which had been limited to shallow events (less than 3 kilometers), many of these earthquakes were deep (between 5 and 11 kilometers). The location of these preeruptive events at such depth indicates that a larger volume of the volcanic system was affected prior to the 20 March eruption than prior to any of the earlier dome-building eruptions. The depth-time relation between the deep earthquakes and the explosive onset of the eruption is compatible with the upward migration of magmatic gas released from a separate deep reservoir. 相似文献
6.
Reports concerning the ineffectiveness of warfarin rodenticide, used at farms and stores in a rural area about 8 kilometers in diameter near Raleigh, North Carolina, prompted subsequent laboratory testing. All of the 25 Norway rats trapped from the area survived 6 days of no-choice feeding of bait containing 0.025 percent (by weight) warfarin. 相似文献
7.
8.
Bruce JG 《Science (New York, N.Y.)》1977,197(4298):51-53
The Somali Current was measured at approximately maximum strength for the year during the southwest monsoon on 11 and 12 August 1975 (approximately 1(1/2) days) at stations spaced 20 to 35 kilometers apart. These measurements permit a nearly synoptic detailed mapping of the temperature structure of a major western boundary current. 相似文献
9.
Toksöz MN Press F Anderson K Dainty A Latham G Ewing M Dorman J Lammlein D Sutton G Duennebier F Nakamura Y 《Science (New York, N.Y.)》1972,176(4038):1012-1016
Lunar seismic data from artificial impacts recorded at three Apollo seismometers are interpreted to determine the structure of the moon's interior to a depth of about 100 kilomneters. In the Fra Mauro region of Oceanus Procellarum, the moon has a layered crust 65 kilometers thick. The seismic velocities in the upper 25 kilometers are consistent with those in lunar basalts. Between 25 and 65 kilometers, the nearly constant velocity (6.8 kilometers per second) corresponds to velocities in gabbroic and anorthositic rocks. The apparent velocity is high (about 9 kilometers per second) in the lunar mantle immediately below the crust. 相似文献
10.
Legeckis R 《Science (New York, N.Y.)》1977,197(4309):1179-1181
During 1975, westward-moving long waves with a period of about 25 days and a wavelength of 1000 kilometers were observed at a sea surface temperature front in the equatorial Pacific on infrared images obtained by a geostationary environmental satellite system. The absence of these waves during 1976, and the above-average equatorial sea surface temperatures during 1976, may be related to a decrease in the southeasterly trade winds during that year. 相似文献
11.
Expansion and contraction of the sahara desert from 1980 to 1990 总被引:1,自引:0,他引:1
Data from polar-orbiting meteorological satellites have been used to determine the extent of the Sahara Desert and to document its interannual variation from 1980 to 1990. The Sahara Desert ranged from 8,633,000 square kilometers in 1980 to 9,982,000 square kilometers in 1984. The greatest annual north-south latitudinal movement of the southern Saharan boundary was 110 kilometers from 1984 to 1985 and resulted in a decrease in desert area of 724,000 square kilometers. 相似文献
12.
Williams GE 《Science (New York, N.Y.)》1986,233(4760):200-203
A major probable impact structure occurs in middle Proterozoic dacitic volcanics in the Gawler Ranges, central South Australia. The structure has an inner depressed area about 30 kilometers in diameter that contains the Lake Acraman salina, an intermediate depression or ring about 90 kilometers in diameter, and a possible outer ring approximately 160 kilometers in diameter. Outcrops of dacite in Lake Acraman are intensely shattered and contain shatter cones and multiple sets of shock lamellac in quartz grains. The Acraman structure is the largest probable impact structure known in Australia and is the likely source of dacitic ejecta found in late Precambrian marine shales some 300 kilometers to the east. 相似文献
13.
Tropical deforestation and habitat fragmentation in the Amazon: satellite data from 1978 to 1988 总被引:2,自引:0,他引:2
Landsat satellite imagery covering the entire forested portion of the Brazilian Amazon Basin was used to measure, for 1978 and 1988, deforestation, fragmented forest, defined as areas less than 100 square kilometers surrounded by deforestation, and edge effects of 1 kilometer into forest from adjacent areas of deforestation. Tropical deforestation increased from 78,000 square kilometers in 1978 to 230,000 square kilometers in 1988 while tropical forest habitat, severely affected with respect to biological diversity, increased from 208,000 to 588,000 square kilometers. Although this rate of deforestation is lower than previous estimates, the effect on biological diversity is greater. 相似文献
14.
Extensive dark smoke clouds associated with burning oil wells in Kuwait have been seen in data from weather satellites since early February 1991. The smoke is dispersed over a wide area. Variable and strong low level winds have held most of the smoke plume below 3 to 5 kilometers within a few hundred kilometers of the source. Thin veils of smoke have been detected in METEOSAT data as far away as 2000 kilometers east of Kuwait, over southwestern Pakistan at heights between 6 and 7 kilometers. The occasional presence of convective clouds over the fires indicates that some scavenging of the smoke is taking place. 相似文献
15.
The P-wave velocity in shallow crystalline rock decreases systematically from a normal value of about 5.5 kilometers per second 20 kilometers or more from the Garlock and San Andreas faults to less than 3 kilometers per second at distances of less than 2 kilometers from these faults. This lateral velocity gradient closely resembles the shear stress profile. It is proposed that the velocity gradient results from increased fracturing nearer these major strike-slip faults and that this fracturing dominates the response of the shallow crust to tectonic stress. 相似文献
16.
Speeds as high as 44 centimeters per second were observed 200 meters above the ocean bottom under the Gulf Stream at 70 degrees W longitude and were associated with time-dependent motion that had a speed range of 40 centimeters per second and a time scale of about 30 days. These deep current fluctuations appear to be coupled with fluctuations in the surface position of the Stream and with surface and bottom current fluctuations 200 kilometers to the north. 相似文献
17.
Seiff A Kirk DB Sommer SC Young RE Blanchard RC Juergens DW Lepetich JE Intrieri PF Findlay JT Derr JS 《Science (New York, N.Y.)》1979,203(4382):787-790
The four Pioneer Venus entry probes transmitted data of good quality on the structure of the atmosphere below the clouds. Contrast of the structure below an altitude of 50 kilometers at four widely separated locations was found to be no more than a few degrees Kelvin, with slightly warmer temperatures at 30 degrees south latitude than at 5 degrees or 60 degrees north. The atmosphere was stably stratified above 15 or 20 kilometers, indicating that the near-adiabatic state is maintained by the general circulation. The profiles move from near-adiabatic toward radiative equilibrium at altitudes above 40 kilometers. There appears to be a region of vertical convection above the dense cloud deck, which lies at 47.5 to 49 kilometers and at temperature levels near 360 K. The atmosphere is nearly isothermal around 100 kilometers (175 to 180 K) and appears to exhibit a sizable temperature wave between 60 and 70 kilometers. This is where the 4-day wind is believed to occur. The temperature wave may be related to some of the wavelike phenomena seen in Mariner 10 ultraviolet photographs. 相似文献
18.
Synnott SP 《Science (New York, N.Y.)》1981,212(4501):1392
During a detailed search of Voyager 1 frames for additional observations of the satellite 1979J1, two small dark spots were observed in transit in several consecutive wide-angle frames of the Jovian atmosphere. The size, spacing, and motion of these pairs of dark spots indicated that they were the images of 1979J1 and its shadow. Subsequent analysis of images spanning 6 days, however, proved that the satellite observed in these Voyager 1 frames would have been occulted by Jupiter at the times of the Voyager 2 images of 1979J1 and was, therefore, a new satellite. It was subsequently found in transit on Voyager 2 images within 13 degrees of the Voyager 1 prediction. Its period is 7 hours 4 minutes 30 seconds +/- 3 seconds, and its mean distance is 1.793 Jupiter radii (Jupiter radius = 71,400 kilometers). The observable profile appears to be roughly circular with a diameter of 40 kilometers, and the albedo is approximately 0.05, similar to Amalthea's. 相似文献
19.
J Veverka P Thomas A Harch B Clark JF Bell B Carcich J Joseph C Chapman W Merline M Robinson M Malin LA McFadden S Murchie SE Hawkins R Farquhar N Izenberg A Cheng 《Science (New York, N.Y.)》1997,278(5346):2109-2114
On 27 June 1997, the Near Earth Asteroid Rendezvous (NEAR) spacecraft flew within 1212 kilometers of asteroid 253 Mathilde. Mathilde is an irregular, heavily cratered body measuring 66 kilometers by 48 kilometers by 46 kilometers. The asteroid's surface is dark (estimated albedo between 0.035 and 0.050) and similar in color to some CM carbonaceous chondrites. No albedo or color variations were detected. The volume derived from the images and the mass from Doppler tracking of the spacecraft yield a mean density of 1.3 +/- 0.2 grams per cubic centimeter, about half that of CM chondrites, indicating a porous interior structure. 相似文献
20.
Anderson DL 《Science (New York, N.Y.)》1967,157(3793):1165-1173
The C-region of the upper mantle has two transition regions 75 to 90 kilometers thick. In western North America these start at depths of 365 kilometers and 620 kilometers and involve velocity increases of about 9 to 10 percent. The locations of these transition regions, their general shape, and their thicknesses are consistent with, first, the transformation of magnesium-rich olivine to a spinel structure and, then, a further collapse of a material having approximately the properties of the component oxides. The velocity increases associated with each transition region are slightly less than predicted for the appropriate phase change. This can be interpreted in terms of an increasing fayalite content with depth. The location of the transition regions and the seismic velocities in their vicinity supply new information regarding the composition and temperature of the upper mantle. The depths of the transition regions are consistent with temperatures near 1500 degrees C at 365 kilometers and 1900 degrees C at 620 kilometers. 相似文献